Electrode for energy storage devices

ABSTRACT

Provided is an electrode for energy storage devices, which is provided with a collector substrate, an undercoat layer that is formed on at least one surface of the collector substrate and contains carbon nanotubes, and an active material layer that is formed on the surface of the undercoat layer, and wherein the active material layer does not contain a conductive assistant.

TECHNICAL FIELD

The present invention relates to an electrode for an energy storagedevice. More specifically, the invention relates to an energy storagedevice electrode which has an active material layer that contains noconductive additive and an undercoat layer that contains carbonnanotubes (CNTs).

BACKGROUND ART

With the need for smaller sizes, lower weights and higher functionalityin portable electronic devices such as smart phones, digital cameras andhandheld game consoles, the development of high-performance batterieshas been actively promoted in recent years, and demand for secondarybatteries—which can be repeatedly used by charging—is growing rapidly.Lithium-ion secondary batteries in particular, because of their highenergy density and high voltage, and moreover because they have nomemory effect during charging and discharging, are the secondarybatteries currently being most vigorously developed. Electrical cardevelopment is also proceeding apace as part of recent efforts to tackleenvironment problems, and an even higher level of performance is beingdemanded of the secondary batteries that serve as the power source insuch vehicles.

Lithium-ion secondary cells have a structure in which a container housesa positive electrode and a negative electrode capable of intercalatingand deintercalating lithium and a separator interposed between theelectrodes, and is filled with an electrolyte solution (in the case oflithium-ion polymer secondary cells, a gel-like or completely solidelectrolyte instead of a liquid electrolyte solution).

The positive electrode and negative electrode are generally produced byforming a composition which includes an active material capable ofintercalating and deintercalating lithium, an electrically conductivematerial composed primarily of a carbon material, and a binder resininto a layer on a current collector such as copper foil or aluminumfoil. The binder is used for bonding the active material with theconductive material, and moreover for bonding these with the metal foil.Exemplary binders that are commercially available include fluoropolymerswhich are soluble in N-methylpyrrolidone (NMP), such as polyvinylidenefluoride (PVDF), and aqueous dispersions of olefin polymers.

The conductive material included in the positive and negativeelectrodes, also referred to as a “conductive additive,” is an importantmaterial for increasing the electrical conductivity of the activematerial layer and smoothly carrying battery charging and discharging,although there are several drawbacks to the presence of a conductiveadditive in an electrode. For example, the carbon materials used as theconductive additive are often carbon blacks such as acetylene black.However, carbon black has a low bulk density and so the density of acarbon black-containing active material layer decreases, leading to adecline in the volumetric capacity density of the battery. Also, becausepores exist in the carbon black, binder is taken up therein, sometimeslowering adhesion at the current collector/active material layerinterface. In addition, because carbon black is small in size comparedwith the active material, it tends to be shed by the electrode, whichsometimes leads to internal shorting of the battery.

Hence, various problems sometimes arise in secondary batteries which useconductive additive-containing electrodes. However, in cases where, toavoid such a state, no conductive additive is added to the activematerial layer, the density of the active material layer increases, butthe active material layer has a low conductivity and so sufficientcharging and discharging do not take place.

Methods that involve inserting a conductive undercoat layer between thecurrent collector and the active material layer have been developed as away of lowering the battery resistance by increasing adhesion betweenthe current collector and the active material layer and decreasing thecontact resistance. For example, Patent Document 1 discloses the art ofusing, as an undercoat layer, a conductive layer that contains carbon asa conductive filler, and placing this undercoat layer between acurrent-collecting substrate and an active material layer. It has beendemonstrated that, by using a composite current collector having such anundercoat layer, the contact resistance between the current-collectingsubstrate and the active material layer can be reduced, in addition towhich a decrease in capacity during rapid discharge can be suppressedand, moreover, deterioration of the battery can also be suppressed.Similar art is disclosed in Patent Documents 2 and 3 as well.Furthermore, undercoat layers containing carbon nanotubes as aconductive filler are disclosed in Patent Documents 4 and 5.

Using the art disclosed in these patent publications, a lowering of thebattery resistance can be achieved, but because the active materiallayer mentioned in this literature includes a conductive additive, thereis a possibility that the above-described problems will arise.

PRIOR ART DOCUMENTS Patent Documents

Patent Document 1: JP-A H09-097625

Patent Document 2: JP-A 2000-011991

Patent Document 3: JP-A H11-149916

Patent Document 4: WO 2014/042080

Patent Document 5: WO 2015/029949

SUMMARY OF INVENTION Technical Problem

The present invention was arrived at in light of the abovecircumstances. An object of the invention is to provide an energystorage device electrode which prevents the battery resistance fromincreasing even when a conductive additive is not included in the activematerial layer and which, as a result, is able to increase thevolumetric capacity density of the battery.

Solution to Problem

The inventors have conducted extensive investigations aimed at achievingthe above object. As a result, they have discovered that by using anelectrode obtained by forming a conductive additive-free active materiallayer on the undercoat layer of a current-collecting substrate having anundercoat layer which contains carbon nanotubes as a conductivematerial, it is possible to obtain a battery having a higher volumetriccapacity density than when an electrode having a conductiveadditive-containing active material layer is used.

Accordingly, the invention provides the following energy storage deviceelectrode.

-   1. An electrode for an energy storage device, comprising a    current-collecting substrate, a carbon nanotube-containing undercoat    layer formed on at least one side of the current-collecting    substrate, and an active material layer formed on a surface of the    undercoat layer, wherein the active material layer contains no    conductive additive.-   2. The energy storage device electrode of 1 above, wherein the    undercoat layer includes a carbon nanotube dispersant made of a    pendant oxazoline group-containing polymer.-   3. The energy storage device electrode of 2 above, wherein the    polymer is obtained by radical polymerization of an oxazoline    monomer of formula (1) below

wherein X is a polymerizable carbon-carbon double bond-containing group,R¹ to R⁴ are each independently a hydrogen atom, a halogen atom, alinear, branched or cyclic alkyl group of 1 to 5 carbon atoms, an arylgroup of 6 to 20 carbon atoms or an aralkyl group of 7 to 20 carbonatoms.

-   4. The energy storage device electrode of any of 1 to 3 above,    wherein the active material layer contains lithium iron phosphate,    lithium manganate, lithium titanium or titanium oxide as the active    material.-   5. The energy storage device electrode of 4 above, wherein the    titanium oxide is titanium oxide having a bronze-type crystal    structure.-   6. The energy storage device electrode of any of 1 to 5 above,    wherein the active material layer includes a styrene-butadiene    rubber as a binder and a carboxymethylcellulose salt as a thickener.-   7. An energy storage device comprising the energy storage device    electrode of any of 1 to 6 above.-   8. The energy storage device of 7 above which is a lithium-ion    secondary battery.

Advantageous Effects of Invention

Even though the energy storage device electrode of the invention uses anactive material layer that contains no conductive additive, whichadditive is important for smooth charging and discharging of a battery,energy storage devices in which the electrode is employed are able tofully charge and discharge and have a higher volumetric capacity densitythan when a conductive additive is used.

Conductive additives have various drawbacks such as lowering theelectrode density, lowering adhesion of the active material layer andcausing internal shorting due to shedding of the additive from theelectrode. Because the energy storage device electrode of the inventionuses no conductive additives, problems such as a decrease in thevolumetric capacity density of the battery and internal shorting thatarise from the use of such additives can be avoided.

DESCRIPTION OF EMBODIMENTS

[Energy Storage Device Electrode]

The energy storage device electrode of the invention includes acurrent-collecting substrate, a CNT-containing undercoat layer that isformed on at least one side of the current-collecting substrate, and anactive material layer that is formed on a surface of the undercoat layerand contains no conductive additive.

The energy storage device in this invention is exemplified by varioustypes of energy storage devices, including lithium secondary batteries,lithium-ion secondary batteries, proton polymer batteries,nickel-hydrogen batteries, electrical double-layer capacitors, aluminumsolid capacitors, electrolytic capacitors and lead storage batteries.The electrode of the invention is particularly well-suited for use inlithium-ion secondary batteries and electrical double-layer capacitors.

[Current-Collecting Substrate]

The current-collecting substrate may be suitably selected from amongmaterials which have hitherto been used as current-collecting substratesin energy storage device electrodes. For example, use can be made ofthin films of any of the following: aluminum, copper, nickel, gold,silver, and alloys thereof, as well as carbon materials, metal oxides,and conductive polymers. Of these, from the standpoint of electricalconductivity, weight and cost, the use of a metal foil made of aluminumor an aluminum alloy is preferred. The thickness of thecurrent-collecting substrate is not particularly limited, although athickness of from 1 to 100 μm is preferred in this invention.

[Undercoat Layer]

The undercoat layer includes CNTs, and also optionally includes a CNTdispersant and/or a matrix polymer. The undercoat layer is preferablyproduced using a CNT-containing composition (dispersion) which includesCNTs, a solvent and, optionally, a CNT dispersant and/or a matrixpolymer.

Carbon nanotubes are generally produced by an arc discharge process,chemical vapor deposition (CVD), laser ablation or the like. The CNTsused in this invention may be obtained by any of these methods. CNTs arecategorized as single-walled CNTs consisting of a single cylindricallyrolled graphene sheet (abbreviated below as “SWCNTs”), double-walledCNTs consisting of two concentrically rolled graphene sheets(abbreviated below as “DWCNTs”), and multi-walled CNTs consisting of aplurality of concentrically rolled graphene sheets (MWCNTs). Any ofthese may be used in the invention. One of these types of CNT may beused alone, or two or more types may be used in combination.

When SWCNTs, DWCNTs or MWCNTs are produced by the above methods,catalyst metals such as nickel, iron, cobalt or yttrium may remain inthe product, and so purification to remove these impurities is sometimesnecessary. Acid treatment with nitric acid, sulfuric acid or the liketogether with ultrasonic treatment is effective for the removal ofimpurities. However, in acid treatment with nitric acid, sulfuric acidor the like, there is a possibility of the π-conjugated system making upthe CNTs being destroyed and the properties inherent to the CNTs beinglost. It is thus desirable for the CNTs to be purified and used undersuitable conditions.

The average fiber diameter of the CNT, although not particularlylimited, is preferably from 1 to 100 nm, and more preferably from 1 to50 nm.

Specific examples of CNTs that may be used in the invention include CNTssynthesized by the super growth method (available from the New Energyand Industrial Technology Development Organization (NEDO) in theNational Research and Development Agency), eDIPS-CNTs (available fromNEDO in the National Research and Development Agency), the SWNT series(available under this trade name from Meijo Nano Carbon), the VGCFseries (available under this trade name from Showa Denko KK), theFloTube series (available under this trade name from CNano Technology),AMC (available under this trade name from Ube Industries, Ltd.), theNANOCYL NC7000 series (available under this trade name from NanocylS.A.), Baytubes (available under this trade name from Bayer),GRAPHISTRENGTH (available under this trade name from Arkema), MWNT7(available under this trade name from Hodogaya Chemical Co., Ltd.) andHyperion CNT (available from Hyperion Catalysis International).

The solvent is not particularly limited, provided it is one that hashitherto been used in the preparation of CNT-containing compositions.Illustrative examples include water, and the following organic solvents:alcohols such as methanol, ethanol, 1-propanol and 2-propanol; etherssuch as tetrahydrofuran (THF), diethyl ether and 1,2-dimethoxyethane(DME); halogenated hydrocarbons such as methylene chloride, chloroformand 1,2-dichloroethane; amides such as N,N-dimethylformamide (DMF),N,N-dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP); ketonessuch as acetone, methyl ethyl ketone, methyl isobutyl ketone andcyclohexanone; aliphatic hydrocarbons such as n-heptane, n-hexane andcyclohexane; aromatic hydrocarbons such as benzene, toluene, xylene andethylbenzene; glycol ethers such as ethylene glycol monoethyl ether,ethylene glycol monobutyl ether and propylene glycol monomethyl ether;and glycols such as ethylene glycol and propylene glycol. Of these, interms of being able to increase the proportion of individually dispersedCNTs, water, NMP, DMF, THF, methanol and isopropanol are preferred.These solvents may be of one type used alone or of two or more typesused in admixture.

The CNT dispersant is preferably made of a pendant oxazolinegroup-containing polymer. The polymer is not particularly limited,although one obtained by radical polymerizing an oxazoline monomerhaving a polymerizable carbon-carbon double bond-containing group at the2 position is preferred. Such oxazoline monomers are exemplified bycompounds of formula (1) below.

In the formula; X is a polymerizable carbon-carbon doublebond-containing group, and R¹ to R⁴ are each independently a hydrogenatom, a halogen atom, a linear, branched or cyclic alkyl group of 1 to 5carbon atoms, an aryl group of 6 to 20 carbon atoms or an aralkyl groupof 7 to 20 carbon atoms.

The polymerizable carbon-carbon double bond-containing group is notparticularly limited so long as it includes a polymerizablecarbon-carbon double bond. However, an acyclic hydrocarbon group whichincludes a polymerizable carbon-carbon double bond is preferred. Forexample, alkenyl groups of 2 to 8 carbon atoms, such as vinyl, allyl andisopropenyl groups are preferred.

Examples of the halogen atom include fluorine, chlorine, bromine andiodine atoms. Examples of the alkyl group include methyl, ethyl,n-propyl, isopropyl, n-butyl, s-butyl, t-butyl and n-pentyl groups.Examples of the aryl group include phenyl, xylyl, tolyl, biphenyl andnaphthyl groups. Examples of the aralkyl group include benzyl,phenylethyl and phenylcyclohexyl groups.

Illustrative examples of the oxazoline monomer of formula (1) include2-vinyl-2-oxazoline, 2-vinyl-4-methyl-2-oxazoline,2-vinyl-4-ethyl-2-oxazoline, 2-vinyl-4-propyl-2-oxazoline,2-vinyl-4-butyl-2-oxazoline, 2-vinyl-5-methyl-2-oxazoline,2-vinyl-5-ethyl-2-oxazoline, 2-vinyl-5-propyl-2-oxazoline,2-vinyl-5-butyl-2-oxazoline, 2-isopropenyl-2-oxazoline,2-isopropenyl-4-methyl-2-oxazoline, 2-isopropenyl-4-ethyl-2-oxazoline,2-isopropenyl-4-propyl-2-oxazoline, 2-isopropenyl-4-butyl-2-oxazoline,2-isopropenyl-5-methyl-2-oxazoline, 2-isopropenyl-5-ethyl-2-oxazoline,2-isopropenyl-5-propyl-2-oxazoline and2-isopropenyl-5-butyl-2-oxazoline. Of these, in terms of availabilityand the like, preferred examples include 2-isopropenyl-2-oxazoline.

The undercoat layer is preferably formed using a CNT-containingcomposition which includes an aqueous solvent. Hence, the oxazolinepolymer is preferably water-soluble. Such a water-soluble oxazolinepolymer may be a homopolymer of an oxazoline monomer of formula (1) or,to further increase the solubility in water, it may be a polymerobtained by the radical polymerization of at least two types of monomer:the above oxazoline monomer and a hydrophilic functionalgroup-containing (meth)acrylic ester monomer.

Specific examples of hydrophilic functional group-containing(meth)acrylic monomers include (meth)acrylic acid, 2-hydroxyethyl(meth)acrylate, methoxy polyethylene glycol (meth)acrylate, amonoesterified product of (meth)acrylic acid and polyethylene glycol,2-aminoethyl (meth)acrylate and salts thereof, sodium (meth)acrylate,ammonium (meth)acrylate, (meth)acrylonitrile, (meth)acrylamide,N-methylol (meth)acrylamide, N-(2-hydroxyethyl) (meth)acrylamide andsodium styrenesulfonate. These may be used singly, or two or more may beused in combination. Of these, methoxy polyethylene glycol(meth)acrylate and a monoesterified product of (meth)acrylic acid andpolyethylene glycol are preferred.

Concomitant use may be made of monomers other than the oxazoline monomerand the hydrophilic functional group-containing (meth)acrylic monomer,provided that doing so does not adversely affect the ability of theoxazoline polymer to disperse CNTs. Illustrative examples of such othermonomers include (meth)acrylic ester monomers such as methyl(meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, 2-ethylhexyl(meth)acrylate, stearyl (meth)acrylate, perfluoroethyl (meth)acrylateand phenyl (meth)acrylate; α-olefin monomers such as ethylene,propylene, butene and pentene; haloolefin monomers such as vinylchloride, vinylidene chloride and vinyl fluoride; styrene monomers suchas styrene and α-methylstyrene; vinyl carboxylate monomers such as vinylacetate and vinyl propionate; and vinyl ether monomers such as methylvinyl ether and ethyl vinyl ether. These may each be used singly, or twoor more may be used in combination.

In terms of further increasing the CNT dispersing ability of theresulting oxazoline polymer, the content of oxazoline monomer in themonomer ingredients used to prepare the oxazoline polymer employed inthe invention is preferably at least 10 wt %, more preferably at least20 wt %, and even more preferably at least 30 wt %. The upper limit inthe content of the oxazoline monomer in the monomer ingredients is 100wt %, in which case a homopolymer of the oxazoline monomer is obtained.

To further increase the water solubility of the resulting oxazolinepolymer, the content of hydrophilic functional group-containing(meth)acrylic monomer in the monomer ingredients is preferably at least10 wt %, more preferably at least 20 wt %, and even more preferably atleast 30 wt %, but is preferably not more than 90 wt %, more preferablynot more than 80 wt %, and even more preferably not more than 70 wt %.

The content of other monomers in the monomer ingredients is in a rangethat does not affect the ability of the resulting oxazoline polymer todisperse CNTs. This content differs according to the type of monomer andthus cannot be strictly specified, but may be suitably set in the rangeof from 5 to 95 wt %, and preferably from 10 to 90 wt %.

The weight-average molecular weight (Mw) of the oxazoline polymer is notparticularly limited, but is preferably from 1,000 to 2,000,000, andmore preferably from 2,000 to 1,000,000. In this invention, the Mw is apolystyrene-equivalent measured value obtained by gel permeationchromatography.

The oxazoline polymer that can be used in this invention may besynthesized by a known radical polymerization of the above monomers ormay be acquired as a commercial product. Illustrative examples of suchcommercial products include Epocros® WS-300 (available from NipponShokubai Co., Ltd.; solids concentration, 10 wt %; aqueous solution),Epocros® WS-700 (Nippon Shokubai Co., Ltd.; solids concentration, 25 wt%; aqueous solution), Epocros® WS-500 (Nippon Shokubai Co., Ltd.; solidsconcentration, 39 wt %; water/1-methoxy-2-propanol solution),Poly(2-ethyl-2-oxazoline) (Aldrich), Poly(2-ethyl-2-oxazoline) (AlfaAesar), and Poly(2-ethyl-2-oxazoline) (VWR International, LLC). When theoxazoline polymer is commercially available as a solution, the solutionmay be used directly as is or may be used after replacing the solventwith a target solvent.

The mixing ratio of the CNTs and the dispersant in the CNT-containingcomposition used in the invention, expressed as a weight ratio, may beset to from about 1,000:1 to about 1:100. The concentration ofdispersant in the CNT-containing composition is not particularlylimited, provided it is a concentration that allows the CNTs to dispersein the solvent. However, the concentration in the composition is set topreferably from about 0.001 to about 30 wt %, and more preferably fromabout 0.002 to about 20 wt %. The concentration of CNTs in theCNT-containing composition varies according to the coating weight of thetarget undercoat layer and the required mechanical, electrical andthermal characteristics, and may be any concentration at which at leastsome portion of the CNTs individually disperse and the undercoat layercan be produced. The concentration of CNTs in the composition ispreferably from about 0.0001 to about 30 wt %, more preferably fromabout 0.001 to about 20 wt %, and even more preferably from about 0.001to about 10 wt %.

Illustrative examples of the matrix polymer include the followingthermoplastic resins: fluoropolymers such as polyvinylidene fluoride(PVDF), polytetrafluoroethylene (PTFE),tetrafluoroethylene-hexafluoropropylene copolymers (P(TFE-HFP)),vinylidene fluoride-hexafluoropropylene copolymers (P(VDF-HFP)) andvinylidene fluoride-chlorotrifluoroethylene copolymers (P(VDF-CTFE)),polyolefin resins such as polyvinylpyrrolidone (PVP),ethylene-propylene-diene terpolymers (EPDM), polyethylene (PE),polypropylene (PP), ethylene-vinyl acetate copolymers (EVA) andethylene-ethyl acrylate copolymers (EEA), polystyrene resins such aspolystyrene (PS), high-impact polystyrene (HIPS), acrylonitrile-styrenecopolymers (AS), acrylonitrile-butadiene-styrene copolymers (ABS),methyl methacrylate-styrene copolymers (MS) and styrene-butadienerubbers (SBR), polycarbonate resins, vinyl chloride resins, polyamideresins, polyimide resins, (meth)acrylic resins such as sodiumpolyacrylate and polymethyl methacrylate (PMMA), polyester resins suchas polyethylene terephthalate (PET), polybutylene terephthalate,polyethylene naphthalate, polybutylene naphthalate, polylactic acid(PLA), poly-3-hydroxybutyric acid, polycaprolactone, polybutylenesuccinate and polyethylene succinate/adipate, polyphenylene etherresins, modified polyphenylene ether resins, polyacetal resins,polysulfone resins, polyphenylene sulfide resins, polyvinyl alcohol(PVA) resins, polyglycolic acids, modified starches, cellulose acetate,carboxymethylcellulose (CMC) and cellulose triacetate, and chitin,chitosan and lignin; the following electrically conductive polymers:polyaniline and emeraldine base (the semi-oxidized form of polyaniline),polythiophene, polypyrrole, polyphenylene vinylene, polyphenylene andpolyacetylene; and the following thermoset or photocurable resins: epoxyresins, urethane acrylate, phenolic resins, melamine resins, urea resinsand alkyd resins. Because it is desirable to use water as the solvent inthe CNT-containing composition of the invention, the matrix polymer ispreferably a water-soluble polymer such as sodium polyacrylate,carboxymethylcellulose sodium, water-soluble cellulose ether, sodiumalginate, polyvinyl alcohol, polystyrene sulfonic acid or polyethyleneglycol. Sodium polyacrylate and carboxymethylcellulose sodium areespecially preferred.

The matrix polymer may be acquired as a commercial product. Illustrativeexamples of such commercial products include sodium polyacrylate (WakoPure Chemical Industries Co., Ltd.; degree of polymerization, 2,700 to7,500), carboxymethylcellulose sodium (Wako Pure Chemical Industries,Ltd.), sodium alginate (Kanto Chemical Co., Ltd.; extra pure reagent),the Metolose® SH Series (hydroxypropylmethyl cellulose, from Shin-EtsuChemical Co., Ltd.), the Metolose® SE Series (hydroxyethylmethylcellulose, from Shin-Etsu Chemical Co., Ltd.), JC-25 (a fully saponifiedpolyvinyl alcohol, from Japan Vam & Poval Co., Ltd.), JM-17 (anintermediately saponified polyvinyl alcohol, from Japan Vam & Poval Co.,Ltd.), JP-03 (a partially saponified polyvinyl alcohol, from Japan Vam &Poval Co., Ltd.) and polystyrenesulfonic acid (from Aldrich Co.; solidsconcentration, 18 wt %; aqueous solution).

The matrix polymer content, although not particularly limited, ispreferably set to from about 0.0001 to about 99 wt %, and morepreferably from about 0.001 to about 90 wt %, of the composition.

The CNT-containing composition used in the invention may include acrosslinking agent that gives rise to a crosslinking reaction with thedispersant used, or a crosslinking agent that is self-crosslinking.These crosslinking agents preferably dissolve in the solvent that isused. The oxazoline polymer crosslinking agent is not particularlylimited, provided it is a compound having two or more functional groupsthat react with oxazoline groups, such as carboxyl, hydroxyl, thiol,amino, sulfinic acid and epoxy groups. A compound having two or morecarboxyl groups is preferred. Compounds which contain functional groups,such as the sodium, potassium, lithium and ammonium salts of carboxylicacids, that, under heating during thin-film formation or in the presenceof an acid catalyst, generate the above functional groups and give riseto crosslinking reactions, may also be used as the crosslinking agent.

Examples of compounds which give rise to crosslinking reactions withoxazoline groups include the metal salts of synthetic polymers such aspolyacrylic acid and copolymers thereof or of natural polymers such asCMC or alginic acid which give rise to crosslink reactivity in thepresence of an acid catalyst, and ammonium salts of these same syntheticpolymers and natural polymers which give rise to crosslink reactivityunder heating. Sodium polyacrylate, lithium polyacrylate, ammoniumpolyacrylate, carboxymethylcellulose sodium, carboxymethylcelluloselithium and carboxymethylcellulose ammonium, which give rise tocrosslink reactivity in the presence of an acid catalyst or underheating conditions, are especially preferred.

These compounds that give rise to crosslinking reactions with oxazolinegroups may be acquired as commercial products. Examples of suchcommercial products include sodium polyacrylate (Wako Pure ChemicalIndustries Co., Ltd.; degree of polymerization, 2,700 to 7,500),carboxymethylcellulose sodium (Wako Pure Chemical Industries, Ltd.),sodium alginate (Kanto Chemical Co., Ltd.; extra pure reagent), Aron®A-30 (ammonium polyacrylate, from Toagosei Co., Ltd.; solidsconcentration, 32 wt %; aqueous solution), DN-800H(carboxymethylcellulose ammonium, from Daicel FineChem, Ltd.), andammonium alginate (Kimica Corporation).

Examples of crosslinking agents that are self-crosslinking includecompounds having, on the same molecule, crosslinkable functional groupswhich react with one another, such as a hydroxyl group with an aldehydegroup, epoxy group, vinyl group, isocyanate group or alkoxy group; acarboxyl group with an aldehyde group, amino group, isocyanate group orepoxy group; or an amino group with an isocyanate group or aldehydegroup; and compounds having like crosslinkable functional groups whichreact with one another, such as hydroxyl groups (dehydrationcondensation), mercapto groups (disulfide bonding), ester groups(Claisen condensation), silanol groups (dehydrative condensation), vinylgroups and acrylic groups.

Specific examples of crosslinking agents that are self-crosslinkinginclude any of the following which exhibit crosslink reactivity in thepresence of an acid catalyst: polyfunctional acrylates,tetraalkoxysilanes, and block copolymers of a blocked isocyanategroup-containing monomer and a monomer having at least one hydroxyl,carboxyl or amino group.

Such crosslinking agents that are self-crosslinking may be acquired ascommercial products. Examples of commercial products includepolyfunctional acrylates such as A-9300 (ethoxylated isocyanuric acidtriacrylate, from Shin-Nakamura Chemical Co., Ltd.), A-GLY-9E(Ethoxylated glycerine triacrylate (ethylene oxide, 9 moles), fromShin-Nakamura Chemical Co., Ltd.) and A-TMMT (pentaerythritoltetraacrylate, from Shin-Nakamura Chemical Co., Ltd.);tetraalkoxysilanes such as tetramethoxysilane (Tokyo Chemical IndustryCo., Ltd.) and tetraethoxysilane (Toyoko Kagaku Co., Ltd.); and blockedisocyanate group-containing polymers such as the Elastron® Series E-37,H-3, H38, BAP, NEW BAP-15, C-52, F-29, W-11P, MF-9 and MF-25K (DKS Co.,Ltd.).

The amount in which these crosslinking agents is added varies accordingto, for example, the solvent to be used, the substrate to be used, therequired viscosity and the required film shape, but is generally from0.001 to 80 wt %, preferably from 0.01 to 50 wt %, and more preferablyfrom 0.05 to 40 wt %, based on the dispersant. These crosslinkingagents, although they sometimes give rise to crosslinking reactions dueto self-condensation, induce crosslinking reactions with the dispersant.In cases where crosslinkable substituents are present in the dispersant,crosslinking reactions are promoted by these crosslinkable substituents.

The following may be added to the CNT-containing composition ascatalysts for accelerating the crosslinking reaction: acidic compoundssuch as p-toluenesulfonic acid, trifluoromethanesulfonic acid,pyridinium p-toluenesulfonic acid, salicylic acid, sulfosalicylic acid,citric acid, benzoic acid, hydroxybenzoic acid and naphthalenecarboxylicacid, and/or thermal acid generators such as2,4,4,6-tetrabromocyclohexadienone, benzoin tosylate, 2-nitrobenzyltosylate and alkyl esters of organic sulfonic acids. The amount ofcatalyst added with respect to the CNT dispersant is preferably from0.0001 to 20 wt %, more preferably from 0.0005 to 10 wt %, and even morepreferably from 0.001 to 3 wt %.

The method of preparing the CNT-containing composition is notparticularly limited. A dispersion may be prepared by the mixture of, inany order: the CNTs, the solvent, and also the optionally useddispersant, matrix polymer and crosslinking agent. The mixture at thistime is preferably dispersion treated. Such treatment enables theproportion of the CNTs that are dispersed to be further increased.Examples of dispersion treatment include mechanical treatment in theform of wet treatment using, for instance, a ball mill, bead mill or jetmill, or in the form of ultrasonic treatment using a bath-type orprobe-type sonicator. Wet treatment using a jet mill and ultrasonictreatment are especially preferred. The dispersion treatment may becarried out for any length of time, although a treatment time of fromabout 1 minute to about 10 hours is preferred, and a period of fromabout 5 minutes to about 5 hours is even more preferred. If necessary,heat treatment may be carried out at this time. When a crosslinkingagent and/or a matrix polymer are used, these may be added followingpreparation of a mixture composed of the dispersant, the CNTs and thesolvent.

The undercoat layer may be formed by applying the above CNT-containingcomposition to at least one side of a current-collecting substrate andthen drying the applied composition in air or under heating. Theundercoat layer may be formed on part of the current-collectingsubstrate surface or may be formed over the entire surface.

In this invention, the coating weight of the undercoat layer per side ofthe current-collecting substrate can generally be set to 1.5 g/m² orless; the advantageous effects of the invention are attainable even at alow coating weight. Accordingly, the coating weight may be set topreferably 0.7 g/m² or less, more preferably 0.5 g/m² or less, and evenmore preferably less than 0.4 g/m². On the other hand, to ensure thatthe undercoat layer functions and to reproducibly obtain batterieshaving excellent characteristics, the coating weight of the undercoatlayer per side of the current-collecting substrate is preferably atleast 0.001 g/m², more preferably at least 0.005 g/m², and even morepreferably at least 0.01 g/m².

The thickness of the undercoat layer is not particularly limited, solong as the above coating weight is satisfied. However, to suppress adecline in the capacity of the resulting device owing to use of theundercoat layer, the thickness is preferably from 0.01 to 10 μm.

The coating weight of the undercoat layer in this invention is the ratioof the undercoat layer weight (g) to the undercoat layer surface area(m²). When the undercoat layer has been formed in a pattern, thissurface area is the surface area of the undercoat layer alone and doesnot include the surface area of the current-collecting substrate thatlies exposed between areas of the patterned undercoat layer.

The weight of the undercoat layer can be determined by, for example,cutting out a test specimen of a suitable size from the undercoat foiland measuring its weight W₀, subsequently peeling the undercoat layerfrom the undercoat foil and measuring the weight W₁ after the undercoatlayer has been removed, and calculating the difference therebetween(W₀-W₁). Alternatively, the weight of the undercoat layer can bedetermined by first measuring the weight W₂ of the current-collectingsubstrate, subsequently measuring the weight W₃ of the undercoat foilafter forming the undercoat layer, and calculating the differencetherebetween (W₃-W₂). The method used to peel off the undercoat layermay involve, for example, immersing the undercoat layer in a solventwhich dissolves the undercoat layer or causes it to swell, and thenwiping off the undercoat layer with a cloth or the like.

The coating weight may be adjusted by a known method. For example, incases where the undercoat layer is formed by coating, the coating weightcan be adjusted by varying the solids concentration of the undercoatlayer-forming coating fluid (CNT-containing composition), the number ofcoating passes or the clearance of the coating fluid delivery opening inthe coater. When one wishes to raise the coating weight, this is done bymaking the solids concentration higher, increasing the number of coatingpasses or making the clearance larger. When one wishes to lower thecoating weight, this is done by making the solids concentration lower,reducing the number of coating passes or making the clearance smaller.

Examples of methods for applying the CNT-containing composition includespin coating, dip coating, flow coating, inkjet coating, spray coating,bar coating, gravure coating, slit coating, roll coating, flexographicprinting, transfer printing, brush coating, blade coating and air knifecoating. From the standpoint of work efficiency and otherconsiderations, inkjet coating, casting, dip coating, bar coating, bladecoating, roll coating, gravure coating, flexographic printing and spraycoating are preferred.

The temperature during drying under applied heat, although notparticularly limited, is preferably from about 50° C. to about 200° C.,and more preferably from about 80° C. to about 180° C.

[Active Material Layer]

The energy storage device electrode of the invention has an activematerial layer on the surface of the above undercoat layer. The activematerial layer can be formed by applying an electrode slurry containingan active material, a binder polymer and, optionally, a thickener or asolvent onto the undercoat layer, then drying the applied slurry in airor under heating. The region where the active material layer is formedshould be suitably selected according to, for example, the cellconfiguration of the device to be used, and may be the entire surface ofthe undercoat layer or some part of that surface. However, when anelectrode assembly having metal tabs and electrodes joined together bywelding (e.g., ultrasonic welding) is to be used in, for example, alaminate cell, in order to leave a welding region, it is preferable toform the active material layer by applying the electrode slurry ontopart of the undercoat layer surface. In laminate cell applications, itis especially preferable to form the active material layer by applyingthe electrode slurry onto all regions of the undercoat layer other thanthe peripheral edge thereof.

Here, various types of active materials that have hitherto been used inenergy storage device electrodes may be used as the active material.

Use may be made of, for example, any of the following as the positiveelectrode active material: chalcogen compounds capable of intercalatingand deintercalating lithium ions, lithium ion-containing chalcogencompounds, polyanion compounds, elemental sulfur, and sulfur compounds.

Illustrative examples of chalcogen compounds capable of intercalatingand deintercalating lithium ions include FeS₂, TiS₂, MoS₂, V₂O₆, V₆O₁₃and MnO₂.

Illustrative examples of lithium ion-containing chalcogen compoundsinclude LiCoO₂, LiMnO₂, LiMn₂O₄, LiMo₂O₄, LiV₃O₈, LiNiO₂ andLi_(x)Ni_(y)M_(1-y)O₂ (wherein M is one or more metal element selectedfrom cobalt, manganese, titanium, chromium, vanadium, aluminum, tin,lead and zinc, 0.05≤x≤1.10, and 0.5≤y≤1.0).

An example of a polyanion compound is lithium iron phosphate (LiFePO₄).

Illustrative examples of sulfur compounds include Li₂S and rubeanicacid.

Of these, it is preferable for the positive electrode active materialused in the invention to be one which includes LiMn₂O₄ or LiFePO₄.

The following may be used as the negative electrode active materialmaking up the negative electrode: alkali metals, alkali metal alloys, atleast one elemental substance selected from among group 4 to 15 elementsof the periodic table which intercalate and deintercalate lithium ions,as well as oxides, sulfides and nitrides thereof, and carbon materialswhich are capable of reversibly intercalating and deintercalatinglithium ions.

Examples of the alkali metals include lithium, sodium and potassium.Examples of the alkali metal alloys include metallic lithium, Li—Al,Li—Mg, Li—Al—Ni, sodium, Na—Hg and Na—Zn.

Examples of the at least one elemental substance selected from amonggroup 4 to 15 elements of the periodic table which intercalate anddeintercalate lithium ions include silicon, tin, aluminum, zinc andarsenic.

Examples of the oxides include titanium oxide (TiO₄), tin silicon oxide(SnSiO₃), lithium bismuth oxide (Li₃BiO₄), lithium zinc oxide (Li₂ZnO₂)and lithium titanate (lithium titanium oxide) (Li₄Ti₅O₁₂).

Examples of the sulfides include lithium iron sulfide (Li_(x)FeS₂(0≤x≤3)) and lithium copper sulfide (Li_(x)CuS (0≤x≤3).

The nitrides are exemplified by lithium-containing transition metalnitrides, such as Li_(x)M_(y)N (M=Co, Ni or Cu; 0≤x≤3, and 0≤y≤0.5), andlithium iron nitride (Li₃FeN₄).

Examples of carbon materials which are capable of reversiblyintercalating and deintercalating lithium ions include graphite, carbonblack, coke, glassy carbon, carbon fibers, carbon nanotubes, andsintered products thereof.

Of these, the negative electrode active material is preferably atitanium-containing oxide such as titanium oxide or lithium titanate. Interms of, for example, the capacity, life and voltage of the device,titanium oxide is preferred. Titanium oxide having a bronze-type crystalstructure (TiO₂(B)) is especially preferred.

A known material may be suitably selected and used as the binderpolymer. Illustrative examples include electrically conductive polymerssuch as PVDF, PVP, PTFE, P(TFE-HFP), P(VDF-HFP), P(VDF-CTFE), PVA,polyimides, EPDM, SBR, CMC, polyacrylic acid (PAA) and polyaniline. Theamount of binder polymer added per 100 parts by weight of the activematerial is preferably from 0.1 to 20 parts by weight, and morepreferably from 1 to 10 parts by weight.

In cases where the electrode slurry has a low viscosity and applicationis difficult, a thickener may be optionally used. The thickener may besuitably selected and used from among known materials, examples of whichinclude the sodium and ammonium salts of CMC. These are available ascommercial products. Specific examples include the same as theabove-mentioned compounds which induce crosslinking reactions withoxazoline groups. The amount of thickener added may be suitably selectedso as to give an electrode slurry suitable for application, with theamount being preferably from 0.1 to 20 parts by weight, and morepreferably from 0.5 to 10 parts by weight, per 100 parts by weight ofthe active material.

The solvent is exemplified by the solvents mentioned above for theCNT-containing composition. The solvent may be suitably selected fromamong these according to the type of binder, although NMP is preferredin the case of water-insoluble binders such as PVDF, and water ispreferred in the case of water-soluble or water-dispersible binders suchas SBR.

The method of applying the electrode slurry is exemplified by the sametechniques as mentioned above for the CNT-containing composition. Thetemperature when drying under applied heat, although not particularlylimited, is preferably from about 50° C. to about 400° C., and morepreferably from about 80° C. to about 150° C.

The thickness of the active material layer, taking into account thebalance between the cell capacity and resistance, is preferably from 10to 500 μm, more preferably from 10 to 300 μm, and even more preferablyfrom 20 to 100 μm.

If necessary, the energy storage device electrode of the invention maybe pressed. Any commonly used method may be employed for pressing,although a mold pressing or roll pressing method is especiallypreferred. The pressing force in roll pressing, although notparticularly limited, is preferably from 0.2 to 3 ton/cm.

[Energy Storage Device]

The energy storage device of the invention is equipped with theabove-described energy storage device electrode. More specifically, itis constructed of at least a pair of positive and negative electrodes, aseparator between these electrodes, and an electrolyte. The energystorage device electrode of the invention is used as at least one ofthese positive and negative electrodes. Constituent members of thedevice other than the foregoing energy storage device electrode, such asthe separator and the electrolyte, may be suitably selected from knownmaterials.

When a known electrode is used as either of the positive and negativeelectrodes, the electrode thus used may be one having the structure ofthe above-described energy storage device electrode, exclusive of theundercoat.

Illustrative examples of the separator include cellulose-basedseparators and polyolefin-based separators.

The electrolyte may be either a liquid or a solid, and moreover may beeither aqueous or non-aqueous, the energy storage device electrode ofthe invention being capable of exhibiting a performance sufficient forpractical purposes even when employed in devices that use a non-aqueouselectrolyte.

The non-aqueous electrolyte is exemplified by a non-aqueous electrolytesolution obtained by dissolving an electrolyte salt in a non-aqueousorganic solvent. Illustrative examples of the electrolyte salt includelithium salts such as lithium tetrafluoroborate, lithiumhexafluorophosphate, lithium perchlorate and lithiumtrifluoromethanesulfonate; quaternary ammonium salts such astetramethylammonium hexafluorophosphate, tetraethylammoniumhexafluorophosphate, tetrapropylammonium hexafluorophosphate,methyltriethylammonium hexafluorophosphate, tetraethylammoniumtetrafluoroborate and tetraethylammonium perchlorate; and lithium imidessuch as lithium bis(trifluoromethanesulfonyl)imide and lithiumbis(fluorosulfonyl)imide. Illustrative examples of non-aqueous organicsolvents include alkylene carbonates such as propylene carbonate,ethylene carbonate and butylene carbonate, dialkyl carbonates such asdimethyl carbonate, methyl ethyl carbonate and diethyl carbonate,nitriles such as acetonitrile, and amides such as dimethylformamide.

The configuration of the energy storage device is not particularlylimited. Cells of various known configurations, such as cylindricalcells, flat wound prismatic cells, stacked prismatic cells, coin cells,flat wound laminate cells and stacked laminate cells may be used.

When used in a coin cell, the energy storage device electrode of theinvention may be die-cut in a specific disk shape and used. For example,a lithium-ion secondary battery may be produced by setting the otherelectrode that has been die-cut to a specific shape on a coin cell capto which a washer and a spacer have been welded, laying an electrolytesolution-impregnated separator of the same shape on top thereof,stacking the energy storage device electrode of the invention on top ofthe separator with the active material layer facing down, placing thecoin cell case and a gasket thereon and sealing the cell with a coincell crimper.

In a stacked laminate cell, use may be made of an electrode assemblyobtained by welding metal tabs at, in electrodes where an activematerial layer has been formed on part or all of the undercoat layersurface, a region of the electrode where the active material layer isnot formed (welding region). In this case, the electrodes making up theelectrode assembly may each consist of a single plate or a plurality ofplates, although a plurality of plates are generally used for both thepositive and the negative electrodes. In cases where welding is carriedout in a region where the undercoat layer is formed and the activematerial layer is not formed, the coating weight of the undercoat layerper side of the current-collecting substrate is 0.1 g/m² or less,preferably 0.09 g/m² or less, and more preferably 0.05 g/m² or less.

The plurality of electrode plates used to form the positive electrodeare preferably stacked in alternation one plate at a time with theplurality of electrode plates that are used to form the negativeelectrode. It is preferable at this time to interpose theabove-described separator between the positive electrode and thenegative electrode.

Metal tabs may be welded to welding regions on the outermost of theplurality of electrode plates, or metal tabs may be welded between thewelding regions on any two adjoining electrode plates. The metal tabmaterial is not particularly limited, provided it is one that iscommonly used in energy storage devices. Examples include metals such asnickel, aluminum, titanium and copper; and alloys such as stainlesssteel, nickel alloys, aluminum alloys, titanium alloys and copperalloys. From the standpoint of welding efficiency, it is preferable forthe tab material to include at least one metal selected from aluminum,copper and nickel. The shape of the metal tabs is preferably foil-like,with the thickness being preferably from about 0.05 mm to about 1 mm.

Known methods for welding together metals may be used as the weldingmethod. Examples include TIG welding, spot welding, laser welding andultrasonic welding. In cases where welding is carried out in a regionwhere an undercoat layer is formed and an active material layer is notformed, because the undercoat layer of the invention is set to a coatingweight that is particularly suitable for ultrasonic welding, it ispreferable to join together the electrodes and the metal tabs byultrasonic welding.

Ultrasonic welding methods are exemplified by a technique in which aplurality of electrode plates are placed between an anvil and a horn,the metal tabs are placed at the welding regions, and welding is carriedout collectively at one time by the application of ultrasonic energy;and a technique in which the electrode plates are first welded together,following which the metal tabs are welded. In this invention, witheither of these techniques, not only are the metal tabs and electrodeswelded together at the welding regions, the plurality of electrodeplates are ultrasonically welded to each other. The pressure, frequency,output power, treatment time, etc. during welding are not particularlylimited, and may be suitably set while taking into account the materialto be used, the presence or absence of an undercoat layer at the weldingregion, and the coating weight and other characteristics of theundercoat layer.

A laminate cell can be obtained by placing the electrode assemblyproduced as described above within a laminate pack, injecting theelectrolyte solution described above, and subsequently heat sealing.

EXAMPLES

Preparation Examples, Working Examples and Comparative Examples aregiven below to more fully illustrate the invention, although theinvention is not limited by these Examples. In the Examples, theequipment and conditions used when preparing specimens and analyzingtheir properties were as follows.

-   (1) Probe-type ultrasonicator:

Equipment: UIP1000 (Hielscher Ultrasonics GmbH)

-   (2) Wire bar coater:

Equipment: PM-9050MC (SMT Co., Ltd.)

-   (3) Charge/discharge measurement system:

Equipment: TOSCAT 3100 (Toyo System Co., Ltd.)

-   (4) Homogenizing disperser

Equipment: T. K. Robomix (with Homogenizing Disperser model 2.5 (32 mmdia.)), from Primix Corporation

-   (5) Thin-film spin-type high-speed mixer

Equipment: Filmix model 40 (Primix Corporation)

-   (6) Planetary centrifugal mixer

Equipment: Thinky Mixer ARE-310 (Thinky)

-   (7) Roll press

Equipment: HSR-60150H ultra-small desktop hot roll press (HohsenCorporation)

-   (8) Coin Cell Crimper

Equipment: CR 2032 manual coin cell crimper (Hohsen Corporation)

[1] Production of Undercoat Foil

Preparation Example 1

First, 2.0 g of the oxazoline polymer-containing aqueous solutionEpocros WS-700 (Nippon Shokubai Co., Ltd.; solids concentration, 25 wt%; Mw=4×10⁴; oxazoline group content, 4.5 mmol/g) as the dispersant wasmixed together with 47.5 g of distilled water, and 0.5 g of MWCNTs(NC7000, from Nanocyl; diameter, 10 nm) was mixed therein. The resultingmixture was ultrasonically treated for 30 minutes at room temperatureusing a probe-type ultrasonicator, thereby giving a blackMWCNT-containing dispersion in which MWCNTs were uniformly dispersed andwhich was free of precipitate.

Next, 0.7 g of the ammonium polyacrylate-containing aqueous solutionAron A-30 (solids concentration, 31.6 wt %; from Toagosei Co., Ltd.),0.2 g of sodium alginate (Kanto Chemical Co., Ltd.; extra pure reagent)and 49.1 g of distilled water were added to 50 g of the resultingMWCNT-containing dispersion and stirring was carried out, givingUndercoat Slurry A1.

The resulting Undercoat Slurry A1 was uniformly spread with a wire barcoater (OSP 30, wet film thickness, 30 μm) onto aluminum foil(thickness, 15 μm) as the current-collecting substrate and subsequentlydried for 20 minutes at 150° C. to form an undercoat layer, therebyproducing Undercoat Foil B1. Twenty pieces of the undercoat foil cut todimensions of 5 cm×10 cm were prepared and their weights were measured,following which the weight of the metal foil from which the undercoatlayer had been rubbed off using paper permeated with 0.5 mol/Lhydrochloric acid was measured. The coating weight of the undercoatlayer, as calculated from the weight difference before and after rubbingoff the undercoat layer, was 0.302 g/m².

Preparation Example 2

Aside from using acetylene black (AB) (Denka Black, from Denka Company,Ltd.) instead of MWCNTs, Undercoat Foil B2 was produced in the same wayas in Preparation Example 1. The coating weight of Undercoat Foil B2 wascalculated and found to be 0.329 g/m².

[2] Production of Electrode and Lithium-Ion Battery Using Lithium IronPhosphate (LFP) as Active Material

Working Example 1

The following were mixed together in a homogenizing disperser at 8,000rpm for 5 minutes: 13.9 g of LFP (TATUNG FINE CHEMICALS CO.) as theactive material, 0.550 g of an aqueous dispersion of styrene-butadienerubber (SBR) (48.5 wt %; TRD2001, from JSR Corporation) as the binder,0.267 g of carboxymethylcellulose ammonium salt (NH₄CMC; DN-800H, fromDaicel Corporation) as the thickener, and 15.3 g of deionized water.Next, using a thin-film spin-type high-speed mixer, mixing treatment wascarried out for 60 seconds at a peripheral speed of 25 m/s, in additionto which deaeration was carried out for 30 seconds at 2,200 rpm in aplanetary centrifugal mixer, thereby producing an electrode slurry(solids concentration, 48 wt %; LFP:SBR:NH₄CMC=104:2:2 (weight ratio).

The resulting electrode slurry was uniformly spread (wet film thickness,200 μm) onto Undercoat Foil B1 produced in Preparation Example 1,following which the slurry was dried at 80° C. for 30 minutes and thenat 120° C. for 30 minutes, thereby forming an active material layer onthe undercoat layer. The active material layer was then pressed with aroll press, producing Electrode C1 having an active material layerthickness of 70 μm and a density of 1.86 g/cm³.

The Electrode C1 thus obtained was die-cut in the shape of a 10 mmdiameter disk and the weight was measured, following which the electrodedisk was vacuum dried at 100° C. for 15 hours and then transferred to aglovebox filled with argon. A stack of six pieces of lithium foil (HonjoChemical Corporation; thickness, 0.17 mm) that had been die-cut to adiameter of 14 mm was set on a 2032 coin cell (Hohsen Corporation) capto which a washer and a spacer had been welded, and one piece ofseparator (Celgard 2400) die-cut to a diameter of 16 mm that had beenpermeated for at least 24 hours with an electrolyte solution (KishidaChemical Co., Ltd.; an ethylene carbonate:diethyl carbonate=1:1 (volumeratio) solution containing 1 mol/L of lithium hexafluorophosphate as theelectrolyte) was laid on the foil. The Electrode C1 was then placed ontop thereof with the active material-coated side facing down. One dropof electrolyte solution was deposited thereon, after which the coin cellcase and gasket were placed on top and sealing was carried out with acoin cell crimper. The cell was then left at rest for 24 hours, giving asecondary battery for testing.

Comparative Example 1

The following were mixed together in a homogenizing disperser at 8,000rpm for 5 minutes: 11.1 g of LFP as the active material, 0.458 g of anaqueous dispersion of SBR (48.5 wt %) as the binder, 0.222 g of NH₄CMCas the thickener, 0.444 g of AB (Denka Black, from Denka Company, Ltd.)as a conductive additive, and 17.8 g of deionized water. Next, using athin-film spin-type high-speed mixer, mixing treatment was carried outfor 60 seconds at a peripheral speed of 25 m/s, in addition to whichdeaeration was carried out for 30 seconds at 2,200 rpm in a planetarycentrifugal mixer, thereby producing an electrode slurry (solidsconcentration, 40 wt %; LFP:SBR:NH₄CMC:AB=100:2:2:4 (weight ratio).

The resulting electrode slurry was uniformly spread (wet film thickness,200 μm) onto pure aluminum foil, following which the slurry was dried at80° C. for 30 minutes and then at 120° C. for 30 minutes, therebyforming an active material layer on aluminum foil. The active materiallayer was then pressed with a roll press, producing Electrode C2 havingan active material layer thickness of 52 μm and a density of 1.76 g/cm³.

Using Electrode C2 thus obtained, a secondary battery for testing wasproduced in the same way as in Working Example 1.

Comparative Example 2

Aside from using Undercoat Foil B2 produced in Preparation Example 2instead of Undercoat Foil B1, Electrode C3 and a secondary battery fortesting were successively produced in the same way as in WorkingExample 1. Electrode C3 had a thickness of 71 μm and a density of 1.85g/cm³.

Comparative Example 3

Aside from using pure aluminum foil instead of Undercoat Foil B1,Electrode C4 and a secondary battery for testing were successivelyproduced in the same way as in Working Example 1. Electrode C4 had athickness of 69 μm and a density of 1.86 g/cm³.

The electrode characteristics for the lithium-ion secondary batteriesproduced in Working Example 1 and Comparative Examples 1 to 3 wereevaluated under the following conditions using a charge/dischargemeasurement system. Table 1 shows the electrode density, the theoreticalvolumetric capacity density and the volumetric capacity density duringdischarge at 3 C in Working Example 1 and Comparative Examples 1 to 3.

-   -   Current: Constant-current charging at 0.5 C, constant-current        discharging at 3 C (the capacity of LFP was treated as 170        mAh/g)    -   Cut-off voltage: 4.50 V-2.00 V    -   Temperature: Room temperature

TABLE 1 Conductive Amount of Theoretical Volumetric capacity material ofAB in active Electrode volumetric density during undercoat materiallayer density capacity density discharge at 3 C layer (wt %) (g/cm³)(mAh/cm³) (mAh/cm³) Working Example 1 CNT 0 1.86 304 141 ComparativeExample 1 — 3.7 1.76 276 122 Comparative Example 2 AB 0 1.85 302 107Comparative Example 3 — 0 1.86 304 27

Table 1 shows that, on comparing Comparative Examples 1 and 3, whenacetylene black was included as a conductive additive in the activematerial layer, due to the bulkiness of acetylene black, the electrodedensity decreased, resulting in a decline in the theoretical volumetriccapacity density. However, when a conductive additive was not includedin the active material layer, with a pure aluminum foil, dischargesubstantially did not proceed; as a result, contrary to the theoreticalvolumetric capacity density, the volumetric capacity density duringdischarge at 3 C ended up falling. Here, as demonstrated in WorkingExample 1, when a CNT-containing undercoat layer was incorporated,because a conductive additive was not included in the active materiallayer, the theoretical volumetric capacity density was similar to thatin Comparative Example 3. Moreover, owing to the presence of anundercoat layer, the resistance at the current collector/active materiallayer interface was low and it is apparent that even the volumetriccapacity density during discharge at 3 C was a higher value than inComparative Example 1 where the active material layer included aconductive additive. Moreover, in the case of the acetyleneblack-containing undercoat layer illustrated in Comparative Example 2,although the performance improved relative to pure aluminum foil, thevolumetric capacity density during discharge at 3 C fell short of thatin Comparative Example 1, in which the active material layer included aconductive additive. It is apparent from this that by combining anactive material layer which contains no conductive additive with aCNT-containing undercoat layer, a secondary battery having the maximumvolumetric capacity density can be produced.

[3] Production of Electrode and Lithium-Ion Battery Using TiOz(B) asActive Material

Working Example 2

The following were mixed together in a homogenizing disperser at 6,000rpm for 5 minutes: 9.53 g of TiO₂(B) synthesized by the method describedin J. Electrochem. Soc., 159(1), A49-A54 (2012) as the active material,0.378 g of an aqueous dispersion of SBR (48.5 wt %) as the binder, 0.183g of NH₄CMC as the thickener and 19.9 g of deionized water. Next, usinga thin-film spin-type high-speed mixer, mixing treatment was carried outfor 60 seconds at a peripheral speed of 25 m/s, in addition to whichdeaeration was carried out for 30 seconds at 2,200 rpm in a planetarycentrifugal mixer, thereby producing an electrode slurry (solidsconcentration, 33 wt %; TiO₂(B):SBR:NH₄CMC=104:2:2 (weight ratio).

The resulting electrode slurry was uniformly spread (wet film thickness,200 μm) onto Undercoat Foil B1 produced in Preparation Example 1,following which the slurry was dried at 80° C. for 30 minutes and thenat 120° C. for 30 minutes, thereby forming an active material layer onthe undercoat layer. The active material layer was then pressed with aroll press, producing Electrode C5 having an active material layerthickness of 45 μm and a density of 1.63 g/cm³.

Using Electrode C5 thus obtained, a secondary battery for testing wasproduced in the same way as in Working Example 1.

Comparative Example 4

Aside from using pure aluminum foil instead of Undercoat Foil B1,Electrode C6 and a secondary battery for testing were successivelyproduced in the same way as in Working Example 2. Electrode C6 had athickness of 48 μm and a density of 1.56 g/cm³.

The electrode characteristics for the lithium-ion secondary batteriesproduced in Working Example 2 and Comparative Example 4 were evaluatedunder the following conditions using a charge/discharge measurementsystem. Table 2 shows the electrode density, the theoretical volumetriccapacity density and the volumetric capacity density during discharge at0.5 C in Working Example 2 and Comparative Example 4.

-   -   Current: Constant-current charging and discharging at 0.5 C (the        capacity of TiO₂(B) was treated as 336 mAh/g)    -   Cut-off voltage: 3.00 V-1.00 V    -   Temperature: Room temperature

TABLE 2 Conductive Amount of Theoretical Volumetric capacity material ofAB in active Electrode volumetric density during undercoat materiallayer density capacity density discharge at 0.5 C layer (wt %) (g/cm³)(mAh/cm³) (mAh/cm³) Working Example 2 CNT 0 1.63 526 94 ComparativeExample 4 — 0 1.56 503 60

Table 2 shows that, even when TiO₂(B) was used as the active material,by combining an active material layer that contains no conductiveadditive and a CNT-containing undercoat layer, the battery performanceimproved.

[4] Production of Electrode and Lithium-Ion Battery Using LithiumTitanate (LTO) as Active Material

Working Example 3

The following were mixed together in a homogenizing disperser at 6,000rpm for 5 minutes: 10.7 g of LTO (Toshima Manufacturing Co., Ltd.) asthe active material, 0.424 g of an aqueous dispersion of SBR (48.5 wt %)as the binder, 0.206 g of NH₄CMC as the thickener and 18.7 g ofdeionized water. Next, using a thin-film spin-type high-speed mixer,mixing treatment was carried out for 60 seconds at a peripheral speed of25 m/s, in addition to which deaeration was carried out for 30 secondsat 2,200 rpm in a planetary centrifugal mixer, thereby producing anelectrode slurry (solids concentration, 37 wt %; LTO:SBR:NH₄CMC=104:2:2(weight ratio).

The resulting electrode slurry was uniformly spread (wet film thickness,200 μm) onto Undercoat Foil B1 produced in Preparation Example 1,following which the slurry was dried at 80° C. for 30 minutes and thenat 120° C. for 30 minutes, thereby forming an active material layer onthe undercoat layer. The active material layer was then pressed with aroll press, producing Electrode C7 having an active material layerthickness of 44 and a density of 1.86 g/cm³.

Using Electrode C7 thus obtained, a secondary battery for testing wasproduced in the same way as in Working Example 1.

Comparative Example 5

Aside from using pure aluminum foil instead of Undercoat Foil B1,Electrode C8 and a secondary battery for testing were successivelyproduced in the same way as in Working Example 3. Electrode C8 had athickness of 44 μm and a density of 1.85 g/cm³.

The electrode characteristics for the lithium-ion secondary batteriesproduced in Working Example 3 and Comparative Example 5 were evaluatedunder the following conditions using a charge/discharge measurementsystem. Table 3 shows the electrode density, the theoretical volumetriccapacity density and the volumetric capacity density during discharge at5 C in Working Example 3 and Comparative Example 5.

-   -   Current: Constant-current charging at 0.5 C, constant-current        discharging at 5 C (the capacity of LTO was treated as 175        mAh/g)    -   Cut-off voltage: 3.00 V-1.00 V    -   Temperature: Room temperature

TABLE 3 Conductive Amount of Theoretical Volumetric capacity material ofAB in active Electrode volumetric density during undercoat materiallayer density capacity density discharge at 5 C layer (wt %) (g/cm³)(mAh/cm³) (mAh/cm³) Working Example 3 CNT 0 1.86 313 113 ComparativeExample 5 — 0 1.85 312 100

Table 3 shows that, even when LTO was used as the active material, bycombining an active material layer that contains no conductive additiveand a CNT-containing undercoat layer, the battery performance improved.

[5] Production of Electrode and Lithium-Ion Battery Using LithiumManganate (LMO) as Active Material

Working Example 4

The following were mixed together in a homogenizing disperser at 8,000rpm for 5 minutes: 13.9 g of LMO (Toshima Manufacturing Co., Ltd.) asthe active material, 0.550 g of an aqueous dispersion of SBR (48.5 wt %)as the binder, 0.267 g of NH₄CMC as the thickener and 15.3 g ofdeionized water. Next, using a thin-film spin-type high-speed mixer,mixing treatment was carried out for 60 seconds at a peripheral speed of25 m/s, in addition to which deaeration was carried out for 30 secondsat 2,200 rpm in a planetary centrifugal mixer, thereby producing anelectrode slurry (solids concentration, 48 wt %; LMO:SBR:NH₄CMC=104:2:2(weight ratio).

The resulting electrode slurry was uniformly spread (wet film thickness,100 μm) onto Undercoat Foil B1 produced in Preparation Example 1,following which the slurry was dried at 80° C. for 30 minutes and thenat 120° C. for 30 minutes, thereby forming an active material layer onthe undercoat layer. The active material layer was then pressed with aroll press, producing Electrode C9 having an active material layerthickness of 26 μm and a density of 2.13 g/cm³.

Using Electrode C9 thus obtained, a secondary battery for testing wasproduced in the same way as in Working Example 1.

Comparative Example 6

Aside from using pure aluminum foil instead of Undercoat Foil B1,Electrode C9 and a secondary battery for testing were successivelyproduced in the same way as in Working Example 4. Electrode C9 had athickness of 25 μm and a density of 2.22 g/cm³.

The electrode characteristics for the lithium-ion secondary batteriesproduced in Working Example 4 and Comparative Example 6 were evaluatedunder the following conditions using a charge/discharge measurementsystem. Table 4 shows the electrode density, the theoretical volumetriccapacity density and the volumetric capacity density during discharge at5 C in Working Example 4 and Comparative Example 6.

-   -   Current: Constant-current charging and discharging at 0.05 C        (the capacity of LMO was treated as 148 mAh/g)    -   Cut-off voltage: 4.50 V-3.00 V    -   Temperature: Room temperature

TABLE 4 Conductive Amount of Theoretical Volumetric capacity material ofAB in active Electrode volumetric density during undercoat materiallayer density capacity density discharge at 0.05 C layer (wt %) (g/cm³)(mAh/cm³) (mAh/cm³) Working Example 4 CNT 0 2.13 304 62 ComparativeExample 6 — 0 2.22 317 0

Table 4 shows that, even when LMO was used as the active material, bycombining an active material layer that contained no conductive additiveand a CNT-containing undercoat layer, the battery performance improved.

The invention claimed is:
 1. An electrode for an energy storage device,comprising a current-collecting substrate, a carbon nanotube-containingundercoat layer formed on at least one side of the current-collectingsubstrate, and an active material layer formed on a surface of theundercoat layer, wherein the active material layer contains noconductive additive, wherein the undercoat layer includes a carbonnanotube dispersant made of a pendant oxazoline group-containingpolymer, wherein the polymer is obtained by radical polymerization of anoxazoline monomer of formula (1) below

wherein X is a polymerizable carbon-carbon double bond-containing group,R¹ to R⁴ are each independently a hydrogen atom, a halogen atom, alinear, branched or cyclic alkyl group of 1 to 5 carbon atoms, an arylgroup of 6 to 20 carbon atoms or an aralkyl group of 7 to 20 carbonatoms.
 2. The energy storage device electrode of any one of claim 1,wherein the active material layer contains lithium iron phosphate,lithium manganate, lithium titanium or titanium oxide as the activematerial.
 3. The energy storage device electrode of claim 2, wherein thetitanium oxide is titanium oxide having a bronze-type crystal structure.4. An energy storage device comprising the energy storage deviceelectrode of claim
 1. 5. The energy storage device of claim 4, whereinthe energy storage device is a lithium-ion secondary battery.